1. Field of Invention
The present invention relates to integrated circuit packaging and, more particularly, to a photoresist process for depositing metal at only selected portions of a metallization pattern pre-existing on ceramic substrate carriers, such as, for example, multilayer ceramic substrates employed as a support for mounting semiconductors devices thereon.
2. Description of the Prior Art
The multilayer ceramic (MLC) technology for fabricating dielectric substrate carriers for integrated circuit/semiconductor package assemblies is well known in the art.
In large scale integrated circuit packages, it is known to provide a ceramic substrate with engineering change (EC) pads in addition to the signal Input/Output (I/O) pads to be solder bonded to active devices, such as semiconductor chips. In actual practice, each of the I/O pads can be associated with and connected to an EC pad, so the EC pads can be used to test the circuitry of the module, the device connection as well as the device itself. In addition, these EC pads provide a means for altering the internal circuitry of the substrate. The concept is described in more detail in U.S. Pat. Nos. 3,726,002, 3,968,193 and 4,221,047 assigned to the assignee of this application, and in the IBM Technical Disclosure Bulletin, Vol, 15 No. 8 January 1973 P. 2575. In use, discrete wires are ultrasonically bonded to the EC pads which provide additional or alternate wiring capable of connecting the various I/O pads of the devices mounted on the substrate.
EC pads are metallurgical areas which individually or collectively require electroplating with a thick film of gold, so called heavy gold. The advantage of gold lies in its well-known ability to provide both low-ohmic contacts and good adhesion between the metallurgical area and the fine interconnecting wire. An electroplating method for forming heavy gold plating on MLC substrates is described in IBM Technical Disclosure Bulletin, Vol. 20 No. 5, October 1977, page 1740. Applying heavy gold on these pads by a plating process has an history of problems. At times, the heavy gold blisters, at others times, adhesion of gold is poor.